1. Field of the Invention
The present invention relates generally to processes for the recovery of alkanols from fluids comprising the alkanol, hydrobromic acid and water, without the formation of excessive amounts of alkyl bromide. More specifically, the invention relates to a distillation system wherein a fluid comprising an alkanol, hydrobromic acid and water is introduced into a distillation column at a point between the bottom and the top of the column to thereby separate the alkanol and the hydrobromic acid without causing the production of a significant amount of alkyl bromide.
2. Discussion of Related Art
It is known that lower alkyl bromides can be readily prepared from the reaction of hydrobromic acid with the corresponding alkanol. For example, U.S. Pat. No. 5,138,110 to Segall describes a process for the continuous production of lower alkyl halides from hydrogen halide and an alkanol in an aqueous acid solution. Indeed, when the acid is hydrobromic acid ("HBr"), and the temperature is elevated, it is difficult to prevent the reaction from occurring; however, lower temperatures and the presence of large amount of water are factors known to retard alkyl bromide formation. The water serves both to dilute the reactants and to drive the following reaction back to the alkanol: EQU HBr+R--OH.rarw..fwdarw.R--Br+H.sub.2 O
Certain operations in the chemical industry result in process streams comprising an aqueous solution of alkanol and HBr, and it is typically necessary to separate the components for recycle. The separation can be accomplished by simple distillation, but a large percentage of the components are lost to the formation of alkyl bromide during the procedure. The alkyl bromide formation occurs both in the distillation flask because of the relatively long residence time at elevated temperature, and probably also in the vapor phase until such time as the components are separated by rectification. In a process wherein the alkanol is methanol, large quantities of volatile methyl bromide can be produced. At one time this was advantageous because the methyl bromide could be advantageously sold as a valuable soil fumigant; however, methyl bromide has recently been classified as an ozone depleting chemical, and its production is becoming highly restricted. Due to the severe impact on production costs which would result if the methyl bromide is collected only to be destroyed by incineration, there is a great need for the development of processes which avoid or minimize alkyl bromide production.
An example of a process well known in the industry which produces streams of aqueous alkanol mixed with HBr, is the manufacture of tetrabromobisphenol-A ("TBBPA"), which is widely used as a flame retardant for synthetic resins. Typically, bisphenol-A is dissolved in an alkanol or aqueous alkanol (generally methanol, ethanol or propanol), and elemental bromine is added thereto and reacted to form TBBPA. One mole of hydrogen bromide is generated for each bromine atom that is inserted onto the aromatic rings. As the bisphenol-A is brominated, it becomes less soluble and begins to drop out of solution. Additional water is commonly added to further promote precipitation. The product is isolated by filtration and further treated to improve purity. For example, it may be washed with water or a water/alkanol solution, and the wash (now comprising HBr and alkanol) may be optionally added to the mother liquor for recycle. While alkanol solvents are the most useful and most economical solvents for use in TBBPA processes, a serious disadvantage of employing an alkanol solvent in a TBBPA process is that at least a portion of the generated HBr reacts with the alkanol to form alkyl bromide during bromination or during subsequent alkanol/HBr separation.
In the TBBPA process, options do exist for minimization of undesirable alkyl bromide formation, both during bromination, and during solvent recovery. With respect to the former, for example, it is well known that mixing water with the alkanol solvent retards alkyl bromide formation to a certain degree; however, high levels of water also reduce solubility and can cause underbrominated species to precipitate with the product, lowering assay. Further, the alkanol must still be separated from the HBr, and separation techniques known in the art cause additional alkyl bromide to be produced.
Other approaches to minimizing alkyl bromide formation during bromination involve avoiding the use of alkanols completely, or removing HBr from the bromination system as it is generated. For example, in U.S. Pat. No. 4,013,728 to Brackenridge it is disclosed that TBBPA may be brominated in aqueous acetic acid rather than in methanol or other alkanol. This approach is not totally satisfactory, however, because it is often difficult to dry the last traces of acetic acid from the precipitate, resulting in an undesirable odor in the product. Further, alkanols are excellent solvents for the TBBPA process, being economical and having many of the necessary solvating properties to provide high quality product. Alternatively, U.S. Pat. No. 5,446,212 to Sanders et al. discloses that alkyl bromide formation may be reduced by removing HBr from the bromination system as it is generated via addition of an oxidizing agent, such as hydrogen peroxide, into the bromination system. Addition of the oxidizing agent results in conversion of HBr to elemental bromine: EQU 2HBr+H.sub.2 O.sub.2 .fwdarw.Br.sub.2 +H.sub.2 O
This approach is not completely satisfactory, however, because it is impractical to convert all of the HBr to bromine. Therefore, while this approach minimizes the amount of HBr in a mother liquor, it does not eliminate its presence therein.
As stated above, another processing step which results in the generation of alkyl bromides is the separation step performed after bromination to recover the alkanol solvent. There have been a number of processes described in the prior art for minimizing alkyl bromide production in the separation step. For example, U.S. Pat. No. 5,395,994 to Williams et al. discloses the neutralization of HBr prior to recovering the alkanol solvent, and German Patent DE 3341037 to Jenkner et al. discloses that the HBr in an ethanol-based mother liquor from the production of hexabromocyclododecane can be neutralized with an alkanolamine prior to distillation. While this may be an effective approach, it requires the use of large amounts of base and the addition of extra water. It is a significant disadvantage of this approach that the base and the water must be disposed of or treated for reuse.
U.S. Pat. No. 4,990,321 to Sato et al. discloses a method for addressing the problem by diluting the solution to less than 10% HBr by weight before distillation. This does reduce alkyl bromide formation, but, again, introduces a significant amount of water to the system which must be removed and disposed of or treated for reuse. Accordingly, there remains a need for a simple, economical process for separating lower boiling alkanols from aqueous solutions of HBr without excessive alkyl bromide formation.